The following method has been known as one of analog-to-digital conversions (A/D conversion) of converting a pixel signal read from a CMOS image sensor to a digital signal. According to this A/D conversion method, a counter takes a count until a signal voltage read from a pixel unit and a reference voltage, which is generated by a reference voltage generator circuit and changes at a predetermined gradient with the time elapse, become the same value. In this way, a video signal is converted to a digital signal (A/D conversion). Specifically, the potential difference between a video signal voltage read from a pixel unit and a reset voltage given as a reference of the video signal voltage is digitized (A/D conversion). However, the foregoing A/D conversion method has the following problem; namely, when the foregoing potential difference is sampled, a thermal noise resulting from sampling occurs.
An A/D conversion method for removing the influence by the foregoing thermal noise is disclosed in the following document.
Document: Woodward. Yanget et al., “An Integrated 800×600 CMOS Imaging system,” 1999 IEEE international Solid-state Circuit Conference. Digest of Technical Papers, 1999 February, pp. 304-305.
The A/D conversion method disclosed in the foregoing document is as follows. Specifically, a reset voltage is A/D-converted using a reference voltage output from a reference voltage generator circuit. Thereafter, a video signal voltage is digitized (A/D conversion) using the reference voltage likewise, and then, the difference between the foregoing two A/D conversion results is obtained as the final A/D conversion value. However, according to this A/D conversion method, a register for storing two-time A/D conversion results is required; for this reason, the circuit scale becomes large.
Moreover, Jpn. Pat. Appln. KOKAI Publication No. 2005-303648 discloses a method of realizing an operation for the difference of two-time A/D conversions using an up-down counter. According to this method, two-time A/D conversions, storage of A/D conversion results and a difference operation are performed using one up-down counter. Specifically, A/D conversion of a reset voltage is carried out in a down-count mode, and thereafter, a counter is changed to an up-count mode, and thus, A/D conversion of a video signal is carried out in the up-down mode. Then, the final counter output value is equivalent to the difference of the video signal and the reset signal. The foregoing method is employed, and thereby, there is no need to provide a register; therefore, the circuit scale is reduced. However, an up-down counter is required, and the number of elements increases compared with the case of using a simple binary counter; as a result, the area of an ADC increases.